AFGHANISTAN (CNN) — Several NATO and Afghan service members were killed Monday when an assailant wearing an Afghan service uniform opened fire on the group, and some of the victims were Americans, NATO's International Security Force officials said. It was unclear if the Americans were U.S. troops, or nonmilitary personnel.
The attack happened late Monday morning in eastern Afghanistan, said Maj. Adam Wojack, a spokesman for the NATO-led force.
The incident appeared to be the latest "green-on-blue" attack, or strike against coalition members by people dressed in police or army uniforms. Assailants conducting similar subterfuge killed dozens of coalition troops in 2012.
On Friday, a coalition contractor in eastern Afghanistan was killed when people wearing Afghan National Army uniforms turned their weapons against ISAF members, NATO said.
The last coalition soldier killed in a "green-on-blue" attack was a Briton, who was slain on January 7. And in the last similar fatal assault on U.S. troops, two Americans were killed October 25.
The coalition has been working to thwart such insider attacks.
Coalition soldiers are required to have a loaded weapon within reach at all times. In addition, the coalition ended training for hundreds of Afghan soldiers last year until the completion of background checks for insurgent links.
Most of the insider attacks are believed to be the result of Afghan soldiers suffering from combat or emotional stress, a Defense Department official told CNN in September after an especially deadly weekend for coalition troops.
Only about 15% of the "green-on-blue" attacks are believed to be the result of insurgent links, and about 10% come from infiltrators not affiliated with the military, the Defense Department official said.
Journalist Masoud Popalzai reported from Kabul, Afghanistan; CNN's Barbara Starr, Wesley Bruer and Lindsey Knight contributed to this report, and Mark Morgenstein wrote it from Atlanta.